Combined Stress Flashcards
On the column show, where will the stress value be greatest?
Point A (at face of column
Pre-stressing a concrete beam primarily adds what type of stress to the section?
Compression
Pre-stressed concrete members usually contain what type of reinforcement?
7-wire steel strand with fy = 250 or 270 ksi.
Post tensioning can be used to reduce the deflection of a beam. (T/F)
True
What is the most common prestressed material?
Concrete.
In a fully prestressed concrete beam, how much of the cross sectional area can be considered effective?
The entire cross sectional area
What is the main difference between pre-tensioned and post-tensioned
concrete?
These terms refer to the state of the concrete during the prestressing process. Pre-tensioning occurs before the concrete cures while post tensioning occurs after the concrete has solidified.
Precast members are typically stressed using which method of tensioning?
Pre-tensioning.
What type of stress is directly introduced by prestressing?
Axial force
What is the main principle that supports the use of the interaction equation?
The principle of superposition
What is meant by the term inverted king post? Or make a sketch of one.
A “King Post Truss” is a simple triangular truss with one vertical member at the center line. In an inverted king post, the lower diagonal chords are prestressed, placing the center strut in compression against the continuous top chord.
For a member to be axially loaded, where must the load be placed?
At the centroid of the section
For the member shown, what are the two forces that
need to be considered when calculating the
combined stress?
The moment Pe and the force P
If the load is placed beyond the middle 1/3 of the section, what happens?
The face away from the load goes into tension
Why should a column be located in the middle 1/3 of a spread footing?
To insure that the whole surface of the footing is bearing on the soil (in compression)
Rafters are commonly analyzed using the moment based on the projected length. What force is neglected by this approach?
The axial force (which is relatively small)
For the system shown below, which is greater, the axial force in the cable or the beam?
Cable
This is always true since
Ah=Bh, but the force in the
cable is Ah+Av
In the system shown above, how would lowering point A effect the load in the
cable?
The force in the cable would increase
In a P-Δ analysis of a compression member, where does the Δ come from?
deflection or initial crookedness
Do post‐tensioned beams have internal stress before they are subjected to live
and dead loads?
yes
Where are pre‐tensioned concrete beams commonly fabricated?
in a factory
For a load‐balanced condition, the shape of the tendon must be the same shape as which diagram?
Moment
What is the effect of attaching the tendon below the neutral axis of the top beam of an inverted king-post truss?
It produces a negative moment which offsets the positive moment from
gravity loads.
What is the advantage of post-tension cables in the plane of a grid shell?
They increase the stiffness of the shell.
In an eccentrically loaded column, when will tensile forces be developed?
When e is greater than or equal to b/6 (outside the middle 1/3)
In a structural glass system such as that used in the entrance to the Renaissance Center in Detroit (the GM building) what is the primary force in the glass?
Tension, it is hung like a chain.
